Inspecting and repairing nuclear boiling water reactors typically consist of manually controlled poles and ropes to manipulate servicing devices and/or positioning of these devices. Remote operation is preferred, and sometimes mandatory, due to the safety risks associated with radiation within the rector. During reactor shut down, servicing of some components require installation of inspection manipulators or devices 30 to 100 feet deep within reactor coolant. The inspection manipulators or devices are connected to a remote control station via umbilical cables. The umbilical cables are costly and cumbersome to install and remove. The individual signal conductors must also be relatively large to improve the signal to noise ratio considering the length of umbilical required. In addition to the high cost, the larger size of the umbilical cables can impede access to locations within the reactor having limited areas of access. Smaller umbilicals provide improved manipulator access and maneuverability. In addition, the umbilical cables tend to be a source of radiation and contamination exposure to personnel from handling. In some cases, fewer manipulator installations can be performed and if a smaller umbilical is utilized on the manipulators, personnel handling and exposure is decreased. The idea of utilizing signal multiplexers is not necessarily new. However, typical multiplexer technology utilizes semiconductors which are susceptible to radiation damage and therefore, may not be utilized with the high radiation levels within the reactor. In addition, conventional signal devices (e.g., ultrasonic probes) do not require a significant number of signal conductors which would create a desire for multiplexing. Although, recent technological advances in some signal devices such as ultrasonic probes and video cameras, require more signal conductors than previous technology which drives the desire for multiplexing. For example, phased array ultrasonic probes require significantly more conductors to operate. The number of conductors can be over 128 times the amount required for conventional ultrasonic probes. With a drive to utilize phased array ultrasonics and thus, more main umbilical signal cables, the desire for multiplexing or switching has increased.
In general, a boiling water nuclear reactor produces electrical power by heating water in a reactor vessel that contains a nuclear fuel core in order to generate steam which is used in turn to drive a steam turbine. FIG. 1 shows a general arrangement of a reactor pressure vessel (RPV) 4. Feedwater is admitted into the RPV 4 via a feedwater inlet (not shown) and a feedwater sparger 6, which is a ring-shaped pipe having suitable apertures for circumferentially distributing the feedwater inside the RPV 4. The feedwater from sparger 6 flows downwardly through the downcomer annulus 8, which is an annular region between a core shroud 2 and RPV 4.
The core shroud 2 is a stainless steel cylinder surrounding the nuclear fuel core, the location of which is generally designated by numeral 9 in FIG. 1. The core is made up of a plurality of fuel bundle assemblies. Each array of fuel bundle assemblies is supported at the top by a top guide and at the bottom by a core plate (neither of which are shown). The core top guide provides lateral support for the top of the fuel assemblies and maintains the correct fuel channel spacing to permit control rod insertion.
The water flows through downcomer annulus 8, around the bottom edge of the shroud and into the core lower plenum 11. The water subsequently enters the fuel assemblies, wherein a boiling boundary layer is established. A mixture of water and steam enters core upper plenum 13 under the shroud head 15. The steam-water mixture than flows through vertical standpipes (not shown) atop the shroud head and enters steam separators (not shown), which separate liquid water from steam. The liquid water then mixes with feedwater in the mixing plenum, which mixture then returns to the core via the downcomer annulus 8. The steam is withdrawn from the RPV 4 via a steam outlet.
The BWR also includes a coolant recirculation system which provides the forced convection flow through the core necessary to attain the required power density. A portion of the water is sucked from the lower end of the downcomer annulus 8 via recirculation water outlet (not visible in FIG. 1) and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies 19 (two of which are shown in FIG. 1) via recirculation water inlets 21. The BWR has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies. The jet pump assemblies are circumferentially distributed around the core shroud 2.
Components and structures in the reactor pressure vessel need to be examined periodically to determine their structural integrity and the need for repair. Ultrasonic inspection is a known technique for detecting and sizing cracks in nuclear reactor components. An inspection area of primary interest is the outside surface of the cylindrical core shroud. However, the core shroud is difficult to access. Installation access is limited to the annular space between the outside of the shroud and the inside of the reactor pressure vessel, between adjacent jet pumps. Scanning operation access is additionally restricted within the narrow space between the shroud and jet pumps. The inspection areas are highly radioactive, and are located under water 50 to 65 feet below the operator's work platform.
An object of this invention is to provide an apparatus and methods that allow the manipulator operations to position devices in various difficult to access areas having relatively high radiation fields, and for several signal generating/receiving devices to be controlled by a main umbilical cable.